1. Field of the Invention
The present invention relates to a sense amplifier circuit for detecting and amplifying two bipolar differential signals. The circuit serves as an interface between the memory portion and the logic portion of a computer.
2. Description of the Prior Art
In general, the voltages V.sub.i1 and V.sub.i2 of two bipolar differential signals can be represented as follows. EQU V.sub.i1 =V.sub.c +V.sub.d EQU V.sub.i2 =V.sub.c -V.sub.d
where V.sub.c is a common direct component of the two signals, and V.sub.d is a differential component of the two signals. In a memory such as an MOS (Metal Oxide Semiconductor) memory of a computer, the value of V.sub.d is, for example, about 0.5 volt (which value is called an MOS level), while, in a logic such as a TTL (Transistor Transistor Logic) inverter of the computer, the value of V.sub.d is about 1.5 volts (which value is called a TTL level). In general, the MOS level is very small compared with the TTL level, so that the signals of the MOS level cannot operate the TTL inverter. As a result, the memory is provided with a sense amplifier circuit for changing an MOS level of two bipolar differential signals to a TTL level thereof. Therefore, the sense amplifier circuit serves as an interface between the memory portion and the logic portion of the computer.
The prior art sense amplifier circuit comprises two or more differential amplifiers to each of which a reference transistor for producing the reference operating voltage of each of the differential amplifiers is connected. Each of the reference operating voltages is preset to be almost constant so that the difference in potential between the two bipolar differential signals can be amplified by the differential amplifiers. When two differential amplifiers are used, the difference in potential between two bipolar differential signals is amplified by a first differential amplifier and, after that, the amplified difference in potential is amplified by a second differential amplifier whose output signals are applied to a logic circuit such as a TTL inverter.
However, in the prior art sense amplifier circuit, the sensing speed is low, since the sensing operation is effected by a two or more stage amplification and the required change of the conductances of the input transistors forming the differential amplifiers is not small due to the constant level of the reference voltages. In addition, the sensing operation is unstable, since the constant reference voltages are changed in response to a change of a power supply voltage. At worst, it may happen that the sensing operation fails.